I want to keep one bitcoins address for donations purposes. How do I make one? because it changes after every transaction and I am not sure what will happen if someone sends bitcoins to the old address. I am using Coinomi Wallet.

in order to prevent Address Reuse[1] most wallets will generate a new address for you each time you want to receive a new payment or your last address was used. but that doesn't mean you lose that previous address. it just becomes hidden so that end users don't reuse that same address by mistake. in other words you can continue using that same address as long as you understand what address reuse is. and Coinomi being a HD wallet means you have a BIP39 Mnemonic phrase which you can store and use whenever you wanted to generate ALL of your addresses and their keys using Coinomi or any other tool.

I use blockchain.info and exchange wallet for my bitcoin. i dont know if coinomi always change your address, but if you want to have permanent wallet you can use blockchain.info or use address from exchange.

What if you use a Wallet from an Exchange.Usually, the Exchange wallet is only one and never changes.Bittrex, Bitfinex, Huobi may be a reference for you.

But you need to be extra cautious, as they might change and if they do yet you didn't check, your assets are gone. Repetitive actions are extremely dangerous as people tend to think that they did things a thousand times and needn't check. Remember, in crypto, there is no undone-button